The long-range objectives of Research Project I are the identification, characterization, and validation of etiologically distinct subtypes or dimensions of learning disabilities. To accomplish these objectives, an extensive psychometric test battery will be administered to a unique sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair has a significant difficulty in the acquisition and use of mathematics, to an independent sample of 50 pairs of identical twins and 50 pairs of fraternal twins in which at least one member of each pair is reading disabled, and to a comparison group of 50 pairs of identical twins and 50 pairs of fraternal twins with a school history of no significant difficulty in mathematics or reading. Resulting data will be used to cross validate measures from Research Projects II and III, assess the genetic and environmental etiologies of deficits in mathematics and reading, estimate the genetic correlation between mathematics and reading disabilities, test hypotheses of differential"genetic etiology as a function of age, gender, and cognitive ability, test the hypothesis that the etiology of deviant scores differs from that of individual differences within the normal range, and assess the structures of genetic and environmental covariance matrices.